A Kentucky county clerk will return to work as soon as Friday after a five-day stint in jail for refusing to issue marriage licences to same-sex couples.

Kim Davis was locked up last Thursday for the boldest act of resistance by a public official yet to the U.S. Supreme Court ruling in June that effectively legalised same-sex marriage across the nation.

Citing "God's authority" and her belief that gay marriage is a sin, Davis stopped issuing all marriage licences.

The apostolic Christian would not say whether she would allow licences to continue to be issued or try to block them again, defying a federal court order that could send her back to jail.

Davis walked out of the Carter County Detention Centre's front door on Tuesday, arm in arm with her lawyer and with Republican presidential candidate Mike Huckabee as thousands of supporters cheered.

Davis will return to work on Friday or Monday, according to an emailed statement from Charla Bansley, a spokeswoman for Liberty Counsel, the Christian law firm representing Davis. The statement did not say whether Davis would allow her office to grant licences.

Her office at the Rowan County Courthouse has opened as scheduled. Three protesters were there, holding signs. Deputy clerk Brian Mason said the office would issue licences to anyone seeking them.

In lifting the contempt order against Davis, US district judge David Bunning said he was satisfied that her deputies were fulfilling their obligation to grant licences to same-sex couples in her absence. But Mr Bunning's said if Davis interferes with the issuance of marriage licences to same-sex couples upon her return, she could go right back to jail.

"I just want to give God the glory. His people have rallied, and you are a strong people," the clerk told the crowd after stepping outside.

Davis did not speak during a brief appearance in front of the news media seconds after her release, however, except to smile and nod when a reporter asked if her decision had been worth it since it landed her in jail.

"Kim cannot and will not violate her conscience," said Mat Staver, founder of the Liberty Counsel, the Christian law firm representing Davis. As for whether she will issue licences, he said: "You'll find out in the near future."

Mr Staver said the licences issued to same-sex couples by Davis's employees last week were not valid since they were not given under Davis's authority. But the Kentucky attorney general's office said it believes otherwise.

On Wednesday, deputy clerk Mason said 10 marriage licences had been issued since Friday, in Davis's absence: eight on Friday and two on Tuesday - and seven of those went to same-sex couples.

Davis, 49, has refused to resign from her 80,000 US dollar job. As an elected official, she can lose her post only if she is defeated for re-election or is impeached by the state General Assembly.

Katherine Franke, a professor at Columbia Law School, said: "The claim she's making is a clear loser. It's a political claim, it's not a legal claim.

"That's why she lost on the district level and the circuit level and she will continue to lose. She's fighting for justice on the level of religious law. But we don't live in a theocracy."